This invention relates to airfoils used for lift, down pressure, stability, control or propulsion of; aircraft, watercraft and other vehicles, fans, turbines, wind generators and other devices that act in and upon fluids, and more particularly to wings (both stationary and rotating), tailplanes, canards, rudders, propellers, rotors, vanes, and impellers.
Inasmuch as this invention is equally applicable to hydrofoils, the term "airfoil" is herein construed to include both airfoils and hydrofoils.
To the present time known airfoils have been designed with a single maximum profile thickness, the object being to retain a smooth streamlined profile. Single maximum thickness dictates that prior art airfoil's chordwise surface profile be single cambered; although single reflexed cambered surfaces with one, two or more convex or concave curves may be used. Prior art teaches that, concave sections and raised protrusions are to be avoided because it is generally believed such features create drag and reduce aerodynamic performance.
Various minor profile protrusions or indentations have been used to aid airfoil performance by inducing early transition or by introducing local separation to energize the boundary layer to delay separation. However, these features are of small size compared to the overall profile of the airfoil and do not alter their general character of a single camber and a single maximum thickness area enclosed by areas of decreasing thickness toward both leading and trailing edges.
In the past century extensive research with single cambered surface airfoils has provided numerous airfoil designs that optimize aerodynamic performance under given conditions. It is generally accepted within the art that enhancement of one aerodynamic quality is possible by trade-off of other aerodynamic qualities. For instance; reduced drag can be achieved while stall performance is sacrificed, higher lift is possible, but usually at the expense of increased drag, stall performance can be improved, but lift or drag performance suffers, overall performance can be improved at some angles-of-attack or at some Reynolds number while accepting reduced performance at others.
An antecedent for the present invention is high aerodynamic performance possible in flight of animals; particularly high attack angles in insect flight.
Some insects can fly at angles of attack up to and beyond 50.degree. (Weis-Fogh, 1973). Further proof of this capability was obtained with insect wings in steady flow (Vogel, 1966). Thin plates with wing shapes stalled at angles of 20.degree., whereas wings did not stall until 50.degree.. Vogel concludes that different dynamic properties of insect wings versus thin plates is due to their morphology. The thin plates lacked four distinguishing features of actual wings. Wings had veins and pleating of the wing membrane, they had rows of hairs on the trailing edge, bristles on the leading edge and microtrichia covering the wing surface.
The inventor believes most of the superior angle-of-attack capabilities of insect wings is due to the veins and pleating which act to reduce boundary layer thickness and delay separation.
Insect wing high stall angle is associated with high-drag coefficient, low-lift coefficient penalties. Insect wings operate at low Reynolds number with relatively large separated flow areas between the veins and pleats forming their surfaces.
The subject invention improves performance by combining high angle-of-attack stall performance with high-lift, low-drag performance. The means by which this is accomplished will become evident from the following explanation and specification with reference to the accompanying drawing figures.